小球随机运动

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        div {
            width: 50px;
            height: 50px;
            background-color: red;
            border-radius: 50%;
            position: absolute;
            left: 0;
            top: 0;
        }
    </style>
</head>

<body>
    <div>

    </div>
    <script>
        var div = document.getElementsByTagName('div')[0];

        var timer = setInterval(move, 30);

        // 浏览器的宽高
        var clientW = window.innerWidth || document.documentElement.clientWidth || document.body.clientWidth;
        var clientH = window.innerHeight || document.documentElement.clientHeight || document.body.clientHeight;

        //div的宽高
        var divW = div.offsetWidth;
        var divH = div.offsetHeight;

        // 小球运动的范围
        var areaX = clientW - divW;
        var areaY = clientH - divH;


        var speedX = ranFun(3,7);
        var speedY = ranFun(4,6);

        function move() {
            var moveX = div.offsetLeft;
            var moveY = div.offsetTop;

            moveX += speedX;
            moveY += speedY;

            if (moveX <= 0 || moveX >= areaX) {
                speedX *= -1;
            }

            if (moveY <= 0 || moveY >= areaY) {
                speedY *= -1;
            }

            div.style.left = moveX + 'px';
            div.style.top = moveY + 'px'
        }


        function ranFun(min, max) {
            return Math.floor(Math.random() * (max - min + 1) + min);
        }
    </script>
</body>

</html>
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